1. Field of the Invention
The invention relates to communication methods and apparatus, in particular, communication methods and apparatus using electric power lines as communications media, and more particularly, powerline communications methods and apparatus using packetized network techniques.
2. Description of the Art
There is an increasing demand for intrabuilding local area networks (LAN's). Intrabuilding electric power distribution lines are potentially useful as a LAN because they are virtually universal in building coverage. However, electric power distribution lines are not designed for communications. As such, they suffer from unpredictable variations in impedance, signal attenuation, and noise. Because of these anomalies, powerline communications systems have been characterized as being relatively low-data-rate, and high-error-rate systems.
In general, in systems with multiaccess channels, such as powerline communications systems, all users share a common transmission medium over which they transmit their packets. Therefore, such systems require resolution of conflicts which arise when more than one demand is placed upon the channel. Whenever a portion of one user's transmission packet overlaps with another user's transmission packet, the two transmission packets collide and obliterate each other. Multi-access broadcast operation must entail either an implicit or explicit method for allowing nodes to share the medium without unduly interfering with each other. These network arbitration methods are generally referred to as Media Access Management, and can be specifically referred to as Media Access Control (MAC).
One MAC arbitration method that is commonly used for communication systems is called carrier sense multiple access (CSMA). CSMA can be highly efficient for environments where the propagation delay is short compared with the transmission time of a packet on the channel. In CSMA schemes, the transmission medium is shared by all user devices, and the medium is allocated to each user for the time required to transmit a single packet. Each user device is connected to the common channel through an interface that listens to all transmissions and receives packets addressed to it.
In general, CSMA requires that each device on the channel sense the carrier due to other devices' transmission and inhibit transmission when the channel is in use, thereby reducing the level of interference caused by the overlapping transmission packets. Packets which either are inhibited or suffer a collision are rescheduled for transmission at a later time according to some rescheduling policy.
Packet rescheduling may incorporate prioritization schemes such as rescheduling a packet transmitted according to a given priority class. Such prioritization schemes can include, for example, the use of pre-assigned time slots for each device on the channel, whereby a given device may transmit if no carrier is sensed on the channel during its time slot. The time slot assignment may or may not be unique. Typically, these MAC schemes also require the use of some definable point at the end of a message to begin the time slot counting. One such point is the sensing of the end-of-carrier (EOC) condition on the channel.
Further, prioritization may be device- or message-based. Device-based priority can tie priority to a device's status in a system hierarchy, to recent transmission activity by that particular device, and the like. Message-based priority can derive a device's rescheduling priority from the content of the message in the packet. Prioritization may range from fully pre-emptive to non-pre-emptive. A non-preemptive priority scheme can require a device to simply "wait its turn" until a pre-assigned device time slot becomes current. At the other end of the spectrum, a fully-preemptive priority scheme can permit a device to preempt the ongoing transmission of another device by intentionally causing a collision.
A often-used form of CSMA is carrier-sense multiple access with collision detection (CSMA-CD) where each transmitting device compares the bit stream it is transmitting to the bit stream it sees on the channel. With CSMA-CD, it is possible for the transceivers to detect interference among several transmissions, including their own, and to abort the transmission of colliding packets.
In powerline applications, the aforementioned CSMA MAC schemes may not provide robust, high-performance broadcasting because of the severely limited bandwidth available for transmissions in the powerline milieu; and because powerline system transmitters typically are unable to receive information while transmitting and, thus, are unable to detect a packet collision or a network anomaly during a transmission. In general, a powerline transmitter can detect collisions only if it does not receive an acknowledgement from its packer's intended recipient.
CSMA-CD is also hampered in a powerline environment by the large disparity between transmit and receive signal strengths, the low impedance placed on the network by an active transmitter, and the substantial noise level inherent in powerline systems. In addition, end-of-carrier cannot be used in powerline communications systems because sensing the end-of-carrier condition is confounded by factors such as delays in receiver filters, media noise, the lack of characterization of the media, and the like.
What is needed, then is a robust, high-performance, low-cost powerline communication method and apparatus.